The following documents are incorporated herein by reference: draft-ietf-sippint-service-example-07; draft-ietf-sip-rfc2543 Session Initiation Protocol; 3GPP TS 29.228; and 3GPP TS 23.218.
The 3GPP standard describes an Internet Protocol (IP) multimedia subsystem (IMS) that comprises the core network (CN) devices that provide IP multimedia services, including audio, video, text, instant messaging, chat and the like, and combinations thereof, delivered over the Internet and/or the public switched telephone network. Voice over IP (VoIP) telephony is supported, but there is no provision for supporting call transfer functionality among VoIP users.
Therefore, there is a need in the art for an improved IP multimedia subsystem that is capable of providing call transfer service for VoIP users.